This invention relates to a process for preparing a soft polyurethane elastomer containing isocyanurate ring, having high resilience and thermal stability, which is useful for a tire filling material.
A "soft elastomer" as used herein means an elastomer having JIS hardness ranging about 10.degree.-60.degree., which is frequently useful for material used under dynamic and high-load conditions, and therefore needs low exothermicity and high resistance to heat (thermal stability). The typical commercially available soft polyurethane elastomer comprises polyoxypropylenepolyol. The prior art soft polyurethane elastomer uses only urethanation curing, and needs a strict control of the equivalent ratio (hereinafter referred to as "ratio NCO/OH") of isocyanate radicals to hydroxy radicals, within about 0.95-1.05 because of entire urethane composition. When the curing reaction incorporates isocyanuration (trimerization by isocyanate radicals), the control of the composition would be facilitated. However, this attempt has not been applied in practice because a soft polyurethane elastomer containing isocyanurate ring has lower thermal stability than that of the entire urethane type soft polyurethane elastomer. It has been believed that the reason for lower stability of the isocyanurate ring-containing elastomer is that tertiary amine which is generally used as catalyst for isocyanuration acts to dissociate thermally the urethane bonding.